The primary function of a pacemaker (cardiac pulse generator) is to sense for intrinsic (natural) events, that is, P-waves and R-waves, and if such are absent to provide stimulation or pacing pulses in either or both the atrium and the ventricle.
A dual chamber pacemaker stimulates a patient's heart in the most physiologically efficient manner when it provides atrioventricular [AV] synchrony. This means that each atrial contraction [P-wave] is followed, after an appropriate AV delay, by a ventricular contraction [R-wave], which is the sequence exhibited by a healthy heart.
Some patients, however, have not only slower than normal heart rates (bradycardia), but also faster than normal heart rates (tachycardia). Atrial tachycardias can be either physiological or pathological. Physiological atrial tachycardia denominates a fast sinus rate (rhythm) wherein the atrium is paced by the sinus node, and is the body's normal response to exercise. Pathological atrial tachycardia is an abnormal rhythm caused by a disease state. A dual chamber pacemaker should react appropriately to bradycardia, and physiological and pathological tachycardias.
In a dual chamber pacemaker, the rate of ventricular pacing pulses [V-pace] which are provided in the absence of sensed intrinsic ventricular activity [V-sense] is determined by the atrial channel. The atrial channel may either sense a P-wave [A-sense] or, in the absence of an A-sense, provide a pacing pulse [A-pace]. When the atrial channel senses a P-wave, the ventricular channel follows, one for one, providing a pacing pulse [V-pace] in the absence of a V-sense. This provides the intended therapy of increased hemodynamic output with increased sinoatrial rate.
However, the dual chamber pacemaker must cope with atrial tachycardias, and the mechanism to do this is the "programmed maximum rate." This maximum rate is usually the upper rate limit both (i) when the ventricular channel is tracking (or following) A-senses in the atrial channel, and (ii) when in the case of a sensor driven, rate responsive pacemaker, the atrial channel is pacing, i.e., providing A-paces. A rate responsive pacemaker has an independent sensor which measures a physical or metabolic parameter. The sinus rate is combined with the sensor indicated rate to determine the atrial pacing rate. This provides increased hemodynamic output for a patient who has sinus node disease.
The behavior of a pacemaker when it reaches the programmed maximum rate is referred to as the "upper rate response". Conventional dual chamber pacemakers employ one or more of a variety of known upper rate responses. The three best known are (i) "block", (ii) "Wenckebach", and (iii) "fallback".
In "block", the pacemaker ignores every second, or third, etc. A-sense depending on the atrial rate, and a V-pace is omitted for each ignored A-sense.
In "Wenckebach", the pacemaker progressively prolongs the AV delay after each A-sense so that a constant ventricular rate is maintained, the maximum rate. There comes a time, depending on the programmed pacemaker intervals and the atrial rate, that an A-sense is ignored and the corresponding V-pace is omitted. This is, in effect, an intermittent "block" response and, as the atrial rate increases, a greater proportion of the A-senses are ignored. This results in a progressively higher degree of "block", resulting in large cardiac cycle-to-cycle variations in the ventricular rate and in intermittent A-V synchrony.
In "fallback", the pacemaker on detecting an atrial rate which exceeds a programmed "tachycardia detection rate", which may be equal to or greater than the maximum rate, progressively lengthens the VV cycle until a programmed lower ventricular rate is obtained. Once this lower rate is obtained, pacing in the VVI mode ensues until the atrial rate drops below the tachycardia rate. AV synchrony is lost for the duration of the "fallback" response.
In U.S. Pat. No. 5,085,215, issued Feb. 4, 1992, to T. A. Nappholz et al, for "Metabolic Demand Driven Rate-Responsive Pacemaker", there is shown a pacemaker which automatically selects either a DDDR or VVIR pacing mode based on an analysis of the patient's intrinsic atrial rate compared to the rate indicated by a metabolic sensor. This comparison provides a classification of such intrinsic atrial rate as either physiological or pathological. This upper atrial rate response, called automatic mode switching [for which AMS is a trademark of Telectronics Pacing Systems, Inc.] provides AV synchrony for normal sinoatrial rates from the minimum to the maximum rate in the DDDR mode, and provides a stable ventricular rate in the VVIR mode, but with the loss of AV synchrony, in the presence of atrial arrhythmias.